Device for guiding a medical imaging probe and method for guiding such a probe

ABSTRACT

A device for guiding a medical imaging probe in order to move the probe close to an anatomical space, the probe having a mechanism for acquiring images of the anatomical space, and the device having at least one sensor mounted to the probe. The device includes a processing mechanism for deducing a rotation of the probe in the space from the signals generated by the sensor and for estimating a plurality of plausible positions of the probe relative to the anatomical space by deducing the rotation of the probe in the space. The processing mechanism is arranged such as to determine the position of the probe relative to the anatomical space from the plausible positions. Also disclosed is a method for guiding the probe.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a National Stage of International Application No.PCT/EP2013/061382 filed Jun. 3, 2013, claiming priority based on FrenchPatent Application No. 12 55130 filed Jun. 1, 2012, the contents of allof which are incorporated herein by reference in their entirety.

The invention relates to a device for guiding a medical imaging probeand to a method for guiding such a probe.

BACKGROUND OF THE INVENTION

So-called “open” operations may prove stressful for a patient. Thus,practitioners are resorting more and more to so-called “minimallyinvasive” operations, during which medical instruments are insertedthrough the skin (percutaneously), or into a natural passage of thepatient (vagina, rectum, auditory canal, etc.), or into an artificialpassage connected to the body of the patient (cannula, artificial vein,trocar, etc.). To this end, practitioners are assisted by images of theorgan or organs in question, the images being taken before or during theintervention.

In urology, in order to detect a possible prostate cancer, it is knownto carry out a prostate biopsy. This involves taking tissue samples fromthe prostate itself, said samples subsequently being analyzed in alaboratory in order to detect the presence of possible cancer cells. Tothis end, the patient is laid on his side or on his back. In the case ofa transrectal biopsy, a medical instrument comprising a needle holderholding a biopsy needle is inserted into the natural passage, namely therectum. By using the medical instrument, the clinician pierces the wallof the colon to reach the prostate and thus take prostate tissuesamples. For the sampling, a two-dimensional ultrasound scan of theprostate, taken during the intervention in the form of a stream ofimages, is typically used by the clinician in order to position theneedle relative to the prostate. However, the use of a two-dimensionalimage to carry out three-dimensional positioning, the relativelyindiscriminate nature of an ultrasound scan as well as the rathersymmetrical shape of the prostate and its mobility often lead to asignificant positioning error of the needle relative to the prostate, sothat the samples cannot in fact be taken regularly or in a targetedfashion, depending on the approach.

In order to help the clinician take the tissue samples at theappropriate positions, there are numerous devices for guiding themedical instrument.

For example, guide devices comprising an articulated arm controlled inorder to move the proximal end of the instrument are known. The positionof the proximal end in the reference frame of the fixed base of thearticulated arm can therefore always be determined. Knowing the positionof the prostate in the reference frame, it is thus possible to know theposition of the instrument relative to the prostate throughout theintervention.

However, such a guide device is particularly bulky.

Moreover, such a guide device only takes into account the movements ofthe instrument. However, the patient may also shift during theintervention, which leads to movement of the prostate. For example, thepatient may not be under local anesthetic and/or he may not berestrained, so that he can shift to find a more comfortable position, orsimply by reflex. Merely the muscle activity of the patient, such as hisrespiratory cycle, may also cause the prostate to move. Furthermore, theprostate is a soft organ, so that the pressure of the instrument, oreven merely the contact of the needle, without its being inserted intothe prostate, is sufficient to move said prostate. In addition, thebladder fills and enlarges during the intervention. It may thereforepress on the prostate so as to move the latter. Hematomas, or liquidaccumulations, caused or not caused by the intervention, may also occurand cause movement of the prostate.

Thus, even if the position of the instrument in the reference frame iscorrectly determined, the prostate may shift so that the cliniciancannot reach the initially intended region with the needle.

Guide devices comprising one or more transmitters (or markers) fixed tothe instrument and a receiver (or detector of the markers), which isarranged in the room in which the patient is, are also known. Forexample, the transmitters generate induced currents which the receivercan detect, which makes it possible to position the transmitters, andtherefore the instrument, in the reference frame of the receiver.Knowing the position of the prostate in the reference frame of thereceiver, it is thus possible to know the position of the instrumentrelative to the prostate during the intervention.

In the same way, however, such a guide device is bulky and takes intoaccount only the movements of the instrument, and in no way the possiblemovements of the prostate: the position of the prostate relative to theinstrument is in fact rapidly displaced, and therefore lost. In othercases, it is possible to add additional sensors on an anatomical volumetargeted by an intervention, in order to resolve this problem, but thisinvasive solution is not applicable to an intervention on the prostate,which is a soft internal organ.

Recently, a new class of guide devices has appeared, allowing theclinician to carry out punctures in the prostate more precisely.Specifically, said guide devices comprise an ultrasound probe providedwith means for three-dimensional image acquisition. Said probe isarranged in the guide device in such a way that a position of the proberelative to the instrument is known, or at least can always bedetermined. For example, the probe is fixed to the instrument.

The probe therefore continuously provides images of the natural passage,the surrounding tissue and the prostate during the movement of theinstrument in the natural passage. These images are then processed inorder to determine the position of the probe, and therefore of theinstrument, with respect to the prostate. Since the images containinformation about the movements of the prostate, the guiding of theinstrument is more precise. Patent Application FR 2 920 961 describessuch a guide device.

However, ultrasound probes acquiring images in three dimensions presentthe drawback of requiring several seconds for the acquisition of animage. Furthermore, since the three-dimensional images contain a largeamount of information, the processing of said images consequently alsotakes a few seconds. It is therefore not possible to know the positionof the probe relative to the prostate rapidly enough after the start ofthe acquisition of a new image, so that the positioning of theinstrument relative to the prostate still remains too imprecise. Inaddition, this approach lengthens the duration of the intervention.

In order to overcome the aforementioned drawbacks, a third class ofguide devices is known, combining localization of the instrument withthe images taken by an ultrasound probe (two dimensions, threedimensions, etc.) and localization by a transmitter/receiver assembly.

The transmitter/receiver assembly makes it possible to rapidly providean approximate position of the probe, and therefore of the instrument,relative to the prostate. This position is used to initialize theprocessing of the images provided by the probe, the processing of theimages then making it possible to refine the position of the proberelative to the prostate.

Thus, the position of the probe, and therefore of the instrument,relative to the prostate is adjusted around an overall positiondetermined rapidly by the transmitter/receiver system.

Such a guide device makes it possible to localize the probe, andtherefore the instrument, very precisely relative to the prostate, butrequires both a probe/image-processing system and a transmitter/receiversystem. In addition, the receiver is often bulky. Furthermore, externalperturbations such as magnetic interference may hamper the localizationof the receivers.

It has therefore been envisioned to replace the transmitter/receiversystem with a sensor carried by the ultrasound probe.

However, such devices prove unreliable, particularly in the event oflarge movements of the prostate between the capture of a reference imageand of a subsequent monitoring image. This is because, with suchdevices, the processing of the images makes it possible to refine theposition provided by the sensor only with local optimization methods.However, if the organ moves significantly because the patient shiftsduring the intervention, the position provided by the sensor may lieoutside what is referred to as the “capture range” of the intendedsolution (i.e. the actual position of the probe relative to theprostate) and it then becomes impossible for a local optimizationcalculation to reach this solution. In this case, these devices thenperform as poorly as the guide devices comprising only a single sensor,since they can provide extremely erroneous and possibly dangerouspositions. Furthermore, it proves necessary to calibrate the guidedevice regularly in order to reset it to a position which is knownrelative to the prostate, which is time-consuming and onerous for theoperator.

OBJECT OF THE INVENTION

It is an object of the invention to at least partly obviate theaforementioned drawbacks.

BRIEF DESCRIPTION OF THE INVENTION

To this end, a device is provided for guiding a medical imaging probe inorder to bring said probe in proximity to an anatomical volume, theprobe comprising means for acquiring images of the anatomical volume,the device comprising at least one sensor fixed to the probe and capableof generating signals representing at least one rotation of the probe inspace. According to the invention, the device comprises a control unitwhich is connected to the probe and comprises means for processing theimages in order to localize the probe relative to the anatomical volume,the processing means being capable of deducing a rotation of the probein space on the basis of the signals generated by the sensor, andcomprising means for estimating a plurality of plausible positions ofthe probe relative to the anatomical volume on the basis of at least thededuction of the rotation of the probe in space, the processing meansbeing arranged in order to determine the position of the probe relativeto the anatomical volume on the basis of a reference image of theanatomical volume, of at least one image acquired by the imageacquisition means and the plausible positions.

Thus, the sensor provides signals making it possible to deduce rapidlyat least one rotation of the probe in space. This rotation is used toinitialize the calculation for determining one or more plausiblepositions of the probe relative to the anatomical volume in question.These plausible positions are then in turn used to initialize theprocessing of the images provided by the acquisition means, which makesit possible to determine the position of the probe relative to theanatomical volume. The estimation of the rotation in space thereforeultimately makes it possible to initialize, by calculating the plausiblepositions, the processing of the images provided by the acquisitionmeans.

By providing this rotation of the probe in space, the processing of theimages is made more robust since it is no longer necessary to determinethe rotation of the probe in space on the basis of the images.Consequently, the processing of the images requires less informationfrom the acquisition means in order to determine the full position ofthe probe relative to the anatomical volume. It therefore becomespossible to work with a smaller amount of information from the imageacquisition means, which makes it possible to reduce the time taken forthe acquisition of the images, and also to shorten the actual processingof the images. By virtue of the invention, the position of the proberelative to the anatomical volume is thus known precisely and rapidly,while obviating a bulky system of the transmitter/receiver type of theprior art, despite the possible movements of the anatomical volume.

Another advantage of the invention is therefore that a smaller amount ofinformation from the acquisition means is necessary compared with guidedevices of the prior art, based only on processing of the images inorder to estimate the position of the probe relative to the anatomicalvolume in question. This makes it possible to speed up the process ofimage acquisition by the image acquisition means, and consequently toknow the position of the probe relative to the anatomical volumesufficiently rapidly after the start of the acquisition of a new image,so that the positioning of the probe relative to the prostate isprecise.

Thus, the combination of a sensor generating signals representing apartial position of the probe, image processing means and estimationmeans makes it possible both to obtain rapid monitoring of the positionof the probe relative to the anatomical volume, by virtue of the sensor,and at the same time to obtain reliable monitoring of said position evenin the event of significant shifts of the targeted anatomical volume, byvirtue of the image estimation means.

Estimating the plausible positions of the probe relative to theanatomical volume on the basis of information about the position of theprobe in space, provided by the sensor, makes it possible to limit theprocessing of the images and avoid situations in which the processing ofthe images does not make it possible to obtain the real position of theprobe relative to the anatomical volume, as with certain devices of theprior art.

According to a preferred embodiment, the sensor is capable of generatingonly signals representing the rotation of the probe in space.

Thus, the estimation means determine the plausible positions of theprobe relative to the anatomical volume only on the basis of informationabout the rotation of the probe in space, i.e. without information aboutthe translation of the probe in space. When a patient is lying downduring the intervention, it is quite likely that he will shift by a fewcentimeters on his bed, but it is far less likely that the anatomicalvolume will turn by more than a few degrees. By virtue of thisembodiment, only information relating to the rotation of the probe isused, i.e. the information which has less likelihood of being greatlymodified during the intervention. The estimation means then make itpossible to estimate the plausible positions of the probe with respectto the anatomical volume, and therefore to estimate the translations ofthe probe in space, which have a high probability of corresponding tothe real position of the probe relative to the anatomical volume.

Here, an anatomical volume is intended to mean both an organ such as thekidney, the breast, the uterus, the thyroid, the liver or the prostate,and a cavity, for example the Douglas' cul-de-sac.

The invention also relates to a method for guiding a medical probe formedical imaging in order to bring the probe in proximity to ananatomical volume, the probe comprising means for acquiring images ofthe anatomical volume, and the method comprising the step of fixing tothe probe at least one sensor capable of generating signals representingat least one rotation of the probe in space. According to the invention,the method comprises the steps of:

processing the signals generated by the sensor in order to deducetherefrom at least one rotation of the probe in space,

estimating a plurality of plausible positions of the probe relative tothe anatomical volume on the basis of at least the deduction of therotation of the probe in space,

processing the images in order to localize the probe relative to theanatomical volume by determining the position of the probe relative tothe anatomical volume on the basis of a reference image of theanatomical volume, of at least one image acquired by the imageacquisition means, and of the plausible positions which have beendetermined.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be understood more clearly in the light of thefollowing description of a nonlimiting particular embodiment of theinvention. Reference is made to the appended figures, in which:

FIG. 1 is a schematic view of a guide device according to the invention,partly inserted into the body of a patient;

FIG. 2 is an enlarged view in section of a part of the guide deviceillustrated in FIG. 1;

FIG. 3 is a diagram illustrating the various steps of a particularembodiment of the method for guiding a probe of the device illustratedin FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The invention is illustrated in its application to a prostate biopsy.This application is, of course, not limiting. Furthermore, the inventionis illustrated in its application to a needle holder. This applicationis also not limiting.

Referring to FIG. 1, the guide device according to the inventioncomprises a medical imaging probe 1, which is illustrated as beinginserted into the rectum 100 of a patient. Here, the guide device isintended to guide the probe 1 in proximity to the prostate 101 of thepatient. The probe 1 comprises means for acquiring images of theprostate 101.

According to one particular embodiment, the probe 1 is rigidly connectedto a medical instrument, here comprising a needle holder 2 for carryingout a biopsy of the prostate 101. The needle holder 2 holds a needle 3.Since the probe 1 is fixed to the needle holder 2, the position of theprobe 1 relative to the needle holder 2 is known.

The guide device comprises a control unit 6, to which the probe 1 isconnected.

Preferably, the device of the invention comprises a screen 12 fordisplaying the images acquired by the probe 1. The display screen 12 isconnected to the control unit 6. Thus, a practitioner can view theimages taken of the prostate 101, or of a particular region of theprostate 101.

According to a preferred embodiment, the control unit 6 comprises means9 for storing the images acquired by the probe.

Referring to FIG. 2, according to a preferred embodiment, theacquisition means comprise a linear ultrasound array 4 for acquiringimages in two dimensions. The acquisition means furthermore comprise inthis case a motor 5 which is arranged in the probe 1 so that rotation ofthe axis of the motor 5 causes rotation of the linear ultrasound array 4about a first axis Y.

Preferably, the motor 5 is a stepper motor, which makes it possible todisplace the linear ultrasound array 4 precisely because there is noaccumulation of an error. The images acquired by the linear ultrasoundarray 4 are therefore of better quality. Furthermore, the motor 5 canthus bring the linear ultrasound array 4 into a precise position, evenover a small range of rotation angle about the first axis Y.

The motor 5 and the linear ultrasound array 4 are connected to thecontrol unit 6, for example by wires.

According to a preferred embodiment, the control unit 6 controls themotor 5 and the linear ultrasound array so that the motor 5 moves thelinear ultrasound array 4 in a restricted rotation range about the firstaxis Y, typically in a rotation range of 20°, when the linear ultrasoundarray 4 is acquiring images.

Preferably, the control unit 6 controls the motor 5 and the linearultrasound array 4 so that the linear ultrasound array 4 acquires asmall number of images, typically three images, when it is rotated bythe motor 5. Preferably, the linear ultrasound array 4 acquires theimages at rotation angles distributed regularly over the rotation rangeabout the first axis Y.

According to a preferred embodiment, the control unit 6 controls themotor 5 and the linear ultrasound array 4 so that the motor 5 brings thelinear ultrasound array into an initial position, referred to as thereference position, and locks the linear ultrasound array 4 in thisreference position for the time taken for the linear ultrasound array 4to acquire at least one image. The motor 5 then drives the linearultrasound array 4 in rotation over a rotation range centered aroundthis reference position, the linear ultrasound array 4 acquiring otherimages during this rotation. Even more preferably, the motor 5 iscontrolled so as to lock the linear ultrasound array 4 in eachpredetermined position in which the linear ultrasound array 4 is meantto acquire an image, for the time taken for the linear ultrasound arrayto acquire said image.

According to a preferred embodiment, the control unit 6 controls themotor 5 and the linear ultrasound array 4 so that, after a first seriesof image acquisitions, the motor 5 brings the linear ultrasound array 4into an initial position for a second series of image acquisitions, saidinitial position being determined at least on the basis of the images ofthe first series.

Thus, the images of the second series are acquired for very particularpositions of the linear ultrasound array 4, in order to focusinformation collection on a region of the prostate 101 to be studied inmore detail, which region is identified with the aid of the first seriesof images. This makes it possible, in particular, to maximize a qualityof localization of the probe 1 relative to said region.

According to the invention, the control unit 6 comprises means 7 forprocessing the images acquired by the image acquisition means.

Furthermore, the device comprises at least one first sensor 10 fixed tothe probe 1 and capable of generating signals which can be used by theprocessing means 7 in order to deduce therefrom at least one rotation ofthe probe 1 in space. The first sensor 10 is, for example, connected tothe control unit 6 by wires. Preferably, the first sensor 10 comprisesat least one gyroscope generating usable signals which make it possibleto calculate an angular position of the probe 1 in space.

Thus, on the basis of the signals generated by the first sensor 10, therotation of the probe 1 in space is determined in real time. In thisway, the determination of the position of the probe 1 relative to theprostate 101, and therefore in this case of the needle holder 2 relativeto the prostate 101, is speeded up. Since the prostate 101 has onlylittle rotational movement of its own, mere knowledge of the rotation ofthe probe 1 in space makes it possible to estimate in an already precisemanner the position of the probe 1 relative to the prostate 101.

Furthermore, the processing means 7 comprise means 20 for estimatingplausible positions of the probe 1, making it possible to calculate aset of plausible positions of the probe with respect to the prostate onthe basis of the angular position of the probe 1 in space, that is tosay on the basis of a partially known position of the probe in space.

Subsequently, on the basis of a reference image and the processing ofthe images acquired by the acquisition means and the plausible positionsof the probe 1 in space, the processing means 7 determine duringoperation the position of the probe 1 relative to the prostate 101. Oncethe position of the probe 1 relative to the needle holder 2 is known,the processing means 7 in this case also make it possible to determinethe position of the needle holder 2 relative to the prostate 101.

Referring to FIG. 3, according to a particular embodiment, the positionof the probe 1 relative to the prostate 101 is determined as follows.

During an initialization sequence, in a first step 201, the acquisitionmeans acquire a first image, referred to as the reference image I_(r),and transmit it to the processing means 7. The reference image is forexample an overall image of the prostate 101, or alternatively an imageof a particular region of the prostate 101. At the same time, the firstsensor 10 generates signals which can be used to determine an angularposition of the probe 1 in real time. Preferably, the processing means 7determine in real time the L angular positions A_(l) of the probe 1during the time which the acquisition of the reference image I_(r) hastaken. In a second step 202, on the basis of the L angular positionsA_(l) determined in the first step 201, the processing means 7 calculatethe average angular position A_(r) of the probe 1 by the followingformula:A _(r)=(1/L)ΣA _(l)

In a third step 203, the processing means 7 store the reference imageI_(r) as well as the average angular position A_(r) f the probe 1,associated with this reference image I_(r).

During a sequence of moving the probe 1, in a first step 301, theacquisition means acquire at least one image I_(s) and transmit it tothe processing means 7. At the same time, the first sensor 10 generatessignals which can be used to determine an angular position of the probe1 in real time. Preferably, the processing means 7 determine in realtime the L angular positions A_(l) of the probe 1 during the time whichthe acquisition of the image I_(s) has taken. In a second step 302, onthe basis of the L angular positions A_(l) determined in the first step301, the processing means 7 calculate the average angular position A_(s)of the probe 1 by the following formula:A _(s)=(1/L)ΣA _(l)

During a third step 303, the processing means 7 determine the relativeangular position A of the probe by the formula:A=(A _(s))⁻¹ A _(r)

It should be recalled that A_(r) is the average angular position ofreference of the probe 1 and A_(s) is the average angular position ofthe probe 1, associated with the sequence of moving the probe 1.

During a fourth step 304, the estimation means 20 implement thefollowing algorithm.

For each integer m between 1 and M, M being an integer predetermined,for example, by a statistical analysis of data of a plurality of initialtests, the estimation means 20 determine T_(m) a plausible position ofthe probe 1 relative to the prostate 101 by the following formula:T _(m)=Mod(A, m)

where Mod is a statistical or cinematic model describing a restrictednumber of plausible positions of the probe 1 with respect to theprostate 101 on the basis of the reference image Ir.

For example, Mod(A,m) may define regular translational sampling for agiven orientation by the following formula:

${{Mod}\;\left( {A,m} \right)} = \left\{ \begin{matrix}{t_{x} = {{\left( {m\mspace{14mu}{modulo}\mspace{14mu} x} \right)*S_{x}} - x_{0}}} \\{t_{y} = {{\left( {\left( {m/x} \right)\mspace{14mu}{modulo}\mspace{14mu} y} \right)*S_{y}} - y_{0}}} \\{t_{z} = {{{m/\left( {x*y} \right)}*S_{z}} - z_{0}}}\end{matrix} \right.$

so that the position T_(m) is then defined byT _(m)(x,y,z)=A(x,y,z)+(t _(x) t _(y) t _(z))^(T)

x₀, y₀, z₀ defining the center of the sampling region,

S_(x), S_(y), S_(z) defining the scaling of the sampling region.

The parameters x, y, z; S_(x), S_(y), S_(z) and x₀, y₀, z₀ are, forexample, determined by a statistical analysis.

The processing means 7 then evaluate a similarity parameter S_(m)between the image I_(s) acquired at the start of the movement sequenceand the reference image I_(r) transformed on the basis of the positionT_(m) according to the formula:S _(m)=Sim(I _(s) ,I _(r) ·T _(m))

where Sim is a measurement function of the distance between two imagesor two subsets of images. Sim is, for example, a measurement function ofthe cross-correlation of two images (or of two subsets of images) whichis determined over the gray levels of the images (or of the elements ofthe two subsets of images).

Thus, during the fourth step 304, on the basis of the information aboutthe rotation of the probe 1 in space and a statistical or cinematicmodel Mod, the estimation means 20 predict a limited but sufficientlyexhaustive number of plausible positions of the probe 1 relative to theprostate 101. For each plausible position, the processing means 7 alsoestimate a similarity parameter between the image acquired at the startof the movement sequence and a transformation of the reference image onthe basis of the associated plausible position. Thus, a similarityparameter S_(m) is associated with each plausible position T_(m.)

During a fifth step 305, the processing means 7 implement the followingalgorithm.

For each integer n between 1 and N, N being an integer predetermined bya statistical analysis of data of a plurality of initial tests, theprocessing means 7 determine the similarity parameter S_(j)corresponding to the n^(th) best result in the list of similarityparameters {S_(l), . . . , S_(m)} calculated during the fourth step 304.

The processing means 7 then carry out a local optimization on the chosensimilarity parameter S_(j) by varying the associated position T_(j)until a maximum similarity parameter S_(n) is found for a position T_(n)which lies in the vicinity of T_(j). The processing means use, forexample, the following formula:

S _(n) , T _(n) =T _(j) ·T _(max)

=arg max_(T) [T _(j)]Sim(I _(s) ,I _(r) ·T _(j) ·T)where T is the optimization variable (the position T is varied in orderto find, around T_(j), a position T_(max) which maximizes thesimilarity).

Local optimization techniques such as the conjugate gradient descentmethod or Powell-Brent method are well known in the prior art and may beused for local optimization on the aforementioned similarity parameterS_(j).

Thus, during the fifth step 305, the processing means 7 test andclassify the plausible positions T_(m) of the probe 1 relative to theprostate 101 which were calculated during the fourth step 304. Thepositions T_(m) associated with a maximum similarity parameter arechosen as candidates which have a high probability of being close to thereal position of the probe 1 with respect to the prostate 101. Thesepositions with a maximum similarity parameter are then studied in moredetail by carrying out a local optimization on the similarity parameterfor each position with a chosen maximum similarity parameter.

Thus, only certain positions T_(m) provided by the estimation means areselected in order to reduce the number of minimizations carried outduring the local optimization.

Next, during a sixth step 306, the processing means 7 determine themaximum similarity parameter S_(k) among all the similarity parametersdetermined during the fifth step 305. The position T_(k) associated withthis maximum similarity parameter S_(k) is the position of the probe 1determined with respect to the prostate 101. This position T_(k) isused, for example, in order to guide the probe 1 relative to theprostate 101.

The image processing consists here of the combination of a calculationof similarity between images with local optimization (based, forexample, on the Powell-Brent or gradient descent method). Theoptimization step consists in starting from one or more initialpositions and varying it or them until the similarity between the twoimages to be anatomically superimposed is locally maximum.

Referring to FIG. 2, according to a preferred embodiment, the devicecomprises a second sensor 11, which is fixed to the probe 1 and capableof generating signals which can be used by the processing means 7 inorder also to deduce a translation of the probe 1 in space therefrom.

On the basis of the signals generated by the first sensor 10 and thesecond sensor 11, the rotation and the translation of the probe 1 inspace are determined in real time, which speeds up even further thedetermination of the position of the probe 1, and therefore of theneedle holder 2, relative to the prostate 101 once an image has beenacquired.

The first sensor 10 and the second sensor 11 are compact and make itpossible to simplify considerably the calculations by the processingmeans 7.

The device according to the invention makes it possible to knowprecisely and rapidly the position of the probe relative to theanatomical volume while obviating a bulky system of thetransmitter/receiver type of the prior art, and to do so despite thepossible movements of the anatomical volume. The device thus makes itpossible to guide the probe precisely and rapidly relative to theanatomical volume, but also here to guide the instrument precisely andrapidly relative to the anatomical volume. Advantageously, the sensor orsensors fixed to the probe are lightweight, of small dimensions, and arearranged in or on the probe.

The device may furthermore be used for numerous applications. Forexample, a practitioner may move the probe and/or the instrument byhimself while being assisted by the images acquired by the probe and therelative position of the probe and the instrument with respect to theanatomical volume, in order to guide the probe and/or the instrument. Asa variant, the device may comprise means for moving the probe and/or theinstrument, the movement means comprising for example an articulatedarm. The control unit will generate control instructions intended forthe movement means in order to control at least one movement of theprobe and/or of the instrument relative to the anatomical volume on thebasis of the images acquired by the probe and the position of the proberelative to the anatomical volume. As a variant, the movement means willbe jointly manipulable with the practitioner.

The invention is not limited to that which has been described above, butrather includes any variant falling within the scope defined by theclaims.

Although the invention is illustrated in an application to a prostatebiopsy, the invention may be used in other applications. For example,the invention may allow treatment of a disease of the prostate, in whichcase an instrument associated with the probe is brought up to theprostate through the perineum or the rectum. The invention may allowpuncture of an anatomical volume of the female genital system, forexample puncture of the uterus, in which case an instrument associatedwith the probe is brought up to the anatomical volume through thevagina. The invention may also allow puncture of another anatomicalvolume such as the kidney, spine, a breast, a lung, etc.

If the probe is intended to be inserted into the body of the patient inorder to reach the intended anatomical volume, the probe may beintroduced through a natural passage such as the rectum, but alsothrough an artificial passage. The probe may also be guided relative tothe intended anatomical volume without being inserted into the body ofthe patient, depending on the application for which the invention isintended. In any event, the invention is entirely independent of thepoint of entry of the natural or artificial passage chosen.

The probe may approach any other anatomical volume than a prostate, suchas a kidney, a breast, or the Douglas' cul-de-sac, etc. The probe maythus be associated with an instrument other than a needle holder, forexample a scalpel, a clamp, a heat probe or an optical fiber, etc. Theprobe may also not be associated with another instrument, in which casethe invention makes it possible to study a particular region of ananatomical volume via the probe. In the case in which the devicecomprises a medical instrument, the medical instrument may be notrigidly connected to the probe. The device will then comprise means forlocalizing the medical instrument relative to the probe, so that theprocessing means determining the position of the probe relative to theanatomical volume can also determine the position of the instrumentrelative to the anatomical volume. The device according to the inventionwill thus make it possible to guide the instrument relative to theanatomical volume. For example, the means for localizing the medicalinstrument relative to the probe will comprise mechanical means forfixing the medical instrument to the probe, such as an articulated arm.

The probe may comprise a different number of sensors and other types ofsensor. Thus, the probe may comprise only one sensor making it possibleto deduce the rotation of the probe in space, this being the mostimportant information. The translational movements of the probe in spacewill then need to be determined in a traditional way by processing theimages acquired by the acquisition means. The first sensor may, forexample, comprise at least one inclinometer and/or at least oneaccelerometer and/or at least one gyrometer. The sensors may be fixed tothe probe without being arranged inside the probe.

The image acquisition means may be other than those described. Forexample, the image acquisition means may be of an optical rather thanultrasound type. For example, the acquisition means may comprise twolinear ultrasound arrays arranged in the probe so as to cross overorthogonally, but will not have a motor. The device may not comprise ascreen for displaying the images taken by the probe. Preferably, thedevice will comprise an interface for communication with a person usingsaid device, such as a display screen and/or audio communication meansand/or haptic feedback means.

Although it has been indicated that the control unit was arranged so asto control the motor and the linear ultrasound array so that, in use,after a first series of image acquisitions, the motor brings the linearultrasound array into an initial position for a second series of imageacquisitions, said initial position being determined at least on thebasis of the images of the first series, the control unit may notcomprise such a function. For certain interventions, it is sometimesnecessary for an image illustrating at least a part of the referencevolume with at least a part of the medical instrument associated withthe probe to be always acquired during a series of image acquisitions.Such an image is in fact very useful for the clinician to orientatehimself. Under these conditions, the initial position for the secondseries of image acquisitions must also be determined so that such animage is acquired during the second series.

The method for determining the position of the probe may be different tothat presented. Only one angular position of the probe may bedetermined, rather than an average angular position of the probe. Otherimage processing steps may be added in order to refine the determinationof the position of the probe 1 relative to the prostate 101. Theintegers N and M may be determined during the initialization sequence.For example, the processing means 7 may continue the image processingafter the step of determining Tk, for example by estimating residualtransformations such as the deformations of the prostate 101 caused bythe probe 1.

As a variant, the method may comprise an additional step of validatingthe result of the calculation of the position of the probe relative tothe anatomical volume. This step consists, for example, in calculating acomplex measure of similarity between the reference image transformed bythe calculated position of the probe relative to the anatomical volumeand the image acquired by the acquisition means, and of validating thissimilarity for example by a threshold method. This will make it possibleto identify an erroneous calculation of the position of the proberelative to the anatomical volume and optionally to warn the user orsuggest to him new strategies for localizing the probe relative to theanatomical volume.

The statistical or cinematic model implemented by the estimation meansmay thus be different to that described. The statistical or cinematicmodel may thus be specific to the type of intervention intended, or evenspecific to the type of category to which the patient belongs. The modelmay thus be a cinematic model of plausible shifts of the probe duringtransrectal access to the prostate, such as that described in PatentApplication FR 2 920 961 A1, a statistical model of the principalcomponent analysis (PCA) type, which makes it possible to describe theprincipal axes of variation and the extent of the variations on thebasis of a statistical analysis, a model constructed according to Bayes'laws intended to identify positions of the probe relative to theanatomical volume with a posteriori maximum probabilities by consideringthe signals generated by the sensor fixed to the probe. The model inquestion will in general be constructed on the basis of a statisticallysufficient number of representative clinical cases. As a variant, theestimation means will comprise calculation means which will provideregular subsampling of the axes of the transformations space fixed tothe probe in space, these not being covered by the signals of the sensorfixed to the probe and/or having a high probability of being exposed tothe shift of the anatomical volume.

The similarity calculation steps may be implemented on the basis of acorrelation coefficient (CC) or the sum of absolute distances (SAD).

The invention claimed is:
 1. A device for guiding a medical imaging probe in order to bring said probe in proximity to an anatomical volume, the probe comprising means for acquiring images of the anatomical volume, the device comprising at least one sensor fixed to the probe and capable of generating signals representing at least one rotation of the probe in space, the device comprises a control unit which is connected to the probe and comprises means for processing the images in order to localize the probe relative to the anatomical volume, the processing means being capable of deducing a rotation of the probe in space on the basis of the signals generated by the sensor, the processing means comprising means for estimating a plurality of plausible positions of the probe relative to the anatomical volume on the basis of at least the deduction of the rotation of the probe in space, the processing means being arranged in order to determine the position of the probe relative to the anatomical volume on the basis of: a reference image of the anatomical volume acquired by the means for acquiring images of the anatomical volume during an initialization phase, at least one image acquired by the image acquisition means during a phase of motion following the initialization phase, and the plausible positions.
 2. The device as claimed in claim 1, wherein the sensor is capable of generating only signals representing the rotation of the probe in space.
 3. The device as claimed in claim 1, wherein the processing means are arranged in order to determine only an angular position of the probe relative to the anatomical volume on the basis of the signals generated by the sensor, and the estimation means are arranged in order to determine a plurality of plausible positions of the probe on the basis of said angular position.
 4. The device as claimed in claim 1, wherein the image acquisition means comprise a linear ultrasound array, and a motor which is arranged so as to drive the linear ultrasound array in rotation about a rotation axis during use.
 5. The device as claimed in claim 4, wherein the control unit is arranged so as to control the motor and the linear ultrasound array so that the motor moves the linear ultrasound array in a restricted rotation range, typically of 20 degrees, about the rotation axis during use.
 6. The device as claimed in claim 4, wherein the control unit is arranged so as to control the motor and the linear ultrasound array so that during use, after a first series of image acquisitions, the motor brings the linear ultrasound array into an initial position for a second series of image acquisitions, said initial position being determined at least on the basis of the images of the first series.
 7. A method for guiding a medical probe for medical imaging in order to bring the probe in proximity to an anatomical volume; the method comprising: providing a probe that acquires images of the anatomical volume, wherein fixed to the probe is at least one sensor capable of generating signals representing at least one rotation of the probe in space and performing the following steps: acquiring a reference image of the anatomical volume during an initialization phase and during a phase of motion following the initialization phase; processing the signals generated by the sensor in order to deduce therefrom at least one rotation of the probe in space; estimating a plurality of plausible positions of the probe relative to the anatomical volume on the basis of at least the deduction of the rotation of the probe in space; processing the images in order to localize the probe relative to the anatomical volume by determining the position of the probe relative to the anatomical volume on the basis of the reference image of the anatomical volume, of at least one image acquired by the probe, and of the plausible positions which have been determined.
 8. The method as claimed in claim 7, wherein the step of estimating a plurality of plausible positions is carried out by a statistical or cinematic model.
 9. The method as claimed in claim 7, wherein the step of estimating a plurality of plausible positions is carried out on the basis of the deduction of only the rotation of the probe in space.
 10. The method as claimed in claim 7, furthermore comprising the step of selecting only certain plausible positions of the probe relative to the anatomical volume in order to initialize the step of processing the images, in order to localize the probe relative to the anatomical volume by determining the position of the probe relative to the anatomical volume.
 11. The method as claimed in claim 7, furthermore comprising the step of validating the position of the probe which has been determined relative to the anatomical volume.
 12. The method as claimed in claim 7, comprising the successive steps of: acquiring at least one reference image of the anatomical volume and determining at least one angular position of the probe in space, associated with the reference image, referred to as a reference angular position; acquiring at least a first image of the anatomical volume and determining at least a first angular position of the probe in space, associated with said image by processing the signals generated by the sensor in order to deduce therefrom the rotation of the probe in space: on the basis of the reference image, the reference angular position, the first image and the first angular position, determining a finite number of positions of the probe relative to the anatomical volume by a statistical or cinematic model, so as to estimate a plurality of plausible positions of the probe relative to the anatomical volume; for each plausible position, calculating a similarity parameter between the image of the anatomical volume and a transformation of the reference image on the basis of said plausible position; with the aid of the similarity parameters calculated in the preceding step, selecting among the plausible positions closest to a real position of the probe relative to the anatomical volume; refining the determination of each position of the probe relative to the anatomical volume by local optimization on each of the similarity parameters associated with the positions; among all the positions thus refined, selecting the position whose associated similarity parameter is maximum after the local optimization step, said position being the position determined for the probe relative to the anatomical volume.
 13. The method as claimed in claim 12, wherein the similarity parameter is calculated on the basis of a cross-correlation over the gray levels between the image of the anatomical volume and the transformation of the reference image.
 14. A medical device, comprising: an imaging probe configured to acquire images of an anatomical volume; a device configured to guide the imaging probe in order to bring the probe in proximity to the anatomical volume; the device comprising at least one sensor fixed to the imaging probe and that generates a signal representing at least one rotation of the probe in space; the device comprises a control unit connected to the probe and a processor in which the images are processed in order to localize the probe relative to the anatomical volume, the processor configured to deduce a rotation of the probe in space on the basis of the signals generated by the sensor and configured to estimate a plurality of plausible positions of the probe relative to the anatomical volume on the basis of at least the deduction of the rotation of the probe in space, and the processor arranged to determine the position of the probe relative to the anatomical volume on the basis of a reference image of the anatomical volume acquired by the imaging probe during an initialization phase, of at least one image acquired by the imaging probe during a phase of motion following the initialization phase, and the plausible positions. 